Getting the router's IP address is easy.
Especially if you're on wireless. Your wireless card/some other way of accessing wirelessly should have an icon in your taskbar indicating a wireless connection. Right-click on that icon and select Status then switch to Support.
But that's only if you're using Window's WZC...(Wireless Zero Configuration, basically a wireless utility for the people who need to connect via such connections.) OR, you can use your wireless's utility to find the IP address.
If not...well, depending on the OS you have, and right now I'm assuming you have Windows XP...you can click Run in the Start Menu and type in CMD, then when the black command prompt comes up...type in IPCONFIG /ALL and the Gateway address will be your router's address.
But be warned, logging into your router by wireless is a suicide run. Do it via a wired ethernet cord.
If you're using Linux or the Apple computers...well, I wouldn't know how to help you in that sort of OS.
And...the reason why you can't forward things to your computer is because you didn't set it up that way. Routers usually have ways of port-forwarding. You need to look for some sort of setting that helps you do that. Since you didn't state the model of your router and DSL modem, there's nothing we can do to help you other than telling you how to figure out what is where and how to do it.